This invention relates generally to systems for welding pipes, and more particularly to a system for welding pipes which automatically compensates for the gravitational effect on the weld which causes undercutting of the weld in the upper section of the weld and concavity of the root of the weld in the lower section of the weld.
Welded pipes must meet ASME (American Society of Mechanical Engineers) specifications and codes. These codes require finished weld geometry, as well as freedom from discontinuities, voids, cracks and porosity. In addition, in welding pipes together, consideration must be given to the pipe orientation due to the effect of gravity on the molten weld puddle. Thus, when two cylindrical pipes are butt welded together in a horizontal orientation, the molten weld puddle at the 12:00 position, i.e. the upper most position, will tend to sag inward, causing an undercut at the outside of the weld. Similarly, at the 6:00 position, i.e., the lower most section of the weld, the inside part of the weld (the weld root) becomes concave and the outside part of the weld bulges outward.
In order to avoid these problems, the pipes are beveled inwardly and the weld is made in many small passes using filler metal. This method keeps the puddle small and when accurately performed, the surface tension of the smaller puddle can be stronger than the force of gravity eliminating the gravitational effect described above. The amount of beveling of the pipe depends on the wall thickness. The thicker the joint is, the more weld metal is required to fill it and thus, more passes and more time are required to complete the joint.
With the present system requiring multiple passes and extreme care in making the weld, highly skilled and experienced welders are required to provide welds with minimum undercutting on the outside of the weld and minimum concaving at the root of the weld.
U.S. Pat. No. 4,121,748 (Yokokawa et al.) discloses an automatic gas pressure welding system which uses positive pressure and heating to butt end pipes together. The system automatically controls the width of the heated areas in conformity with the amount of contraction of the pipes. U.S. Pat. No. 4,733,051 (Nadea et al.) discloses an automatic control system for controlling the penetration of the weld pool. U.S. Pat. No. 5,096,110 (Shmatz et al.) shows an automatic brazing control system which employs water and oxygen pressure as a function of temperature. U.S. Pat. No. 3,614,376 (Boodell) discloses an aluminum tube welding system which applies purging gas under pressure to reduce the concavity of the outer weld surface.
An article in Process Pipe and Tube Welding publication published in 1991 discloses an automatic pressure control system which applies a fixed negative pressure to remove concavity at the 6:00 position.
However, none of the available welding systems apply a continuously variable pressure and vacuum within the pipes during welding which proportionately provides increasing positive pressure from the 9:00 position to the 12:00 position, decreasing positive pressure from the 12:00 position to the 3:00 position, increasing negative pressure (vacuum) from the 3:00 to the 6:00 position and decreasing negative pressure from the 6:00 position to the 9:00 position, as does the present invention. This tends to eliminate all undercutting and concavity throughout the entire circumference of the weld, with the pressure applied within the pipes varying from a maximum positive pressure at 12:00 to a maximum negative pressure at 6:00 and approximately 0 pressure applied at 3:00 and 9:00.
The system of the present invention will drastically reduce the number of passes required in obtaining a weld, the amount of undercutting and concavity of the weld, and the amount of beveling of the surfaces of the pipes being welded together at the weld, thereby reducing the amount of weld material required as well as welding time. In fact, for thin wall pipe sizes less than 0.250 inches, the requirement for a bevel joint is eliminated and the weld can be performed in a single pass using less skilled operators.